1. Field of the Invention
The present invention relates to an optical scanner for an image forming apparatus.
2. Description of the Related Art
A conventional image forming apparatus, such as a printer, is provided with an optical scanner (LSU: laser scanner unit) for writing an electrostatic latent image (image) on a photosensitive drum (object to be scanned) (see Japanese Patent Application Publication No. 2002-328321).
Such an optical scanner has, in a housing thereof, a light source for emitting a laser beam, a polygon mirror for reflecting, while rotating, the laser beam emitted from the light source, a polygon motor for rotating and driving the polygon mirror, and an optical system having a lens or a mirror for guiding the laser beam reflected by the polygon mirror onto a photosensitive drum. The laser beam reflected by the polygon mirror is guided onto the photosensitive drum by the optical system and then scans the photosensitive drum in its axial direction. As a result, an image is written onto the photosensitive drum.
The optical scanner also has a base for controlling an image write start position on the photosensitive drum. A light receiving element is attached to this base. A part of the laser beam reflected by the polygon mirror is reflected by a light guide mirror (mirror member) and enters the light receiving element. As a result, an image write start signal is generated by the base, and the image write start position on the photosensitive drum is determined based on this signal.
Incidentally, the heat of the polygon motor rotating and driving the polygon mirror heats the housing of the optical scanner to relatively high temperature. Therefore, when a member holding the mirror is thermally deformed, the optical path of the reflected light moves, which might incapacitates the optical scanner.
For example, when a holding member holding the light guide mirror is thermally deformed and the optical path of the reflected light on the light guide mirror moves, the laser beam might not be able to enter the light receiving element of the base properly, which makes it difficult to appropriately control the image write start position on the photosensitive drum. As a result, so-called image deviation occurs where the position on the photosensitive drum where the image is written is deviated from the original position, whereby the image quality is reduced. Note that the light guide mirror is usually disposed in a place within the housing where the temperature is likely to be high.